1. Field of the Invention
This invention relates to a light emitting device and, particularly, to a light emitting device with an improved adhesion layer for bonding a light emitting element to its mounting member. Also, this invention relates to a light emitting device with an improved phosphor layer for wavelength-converting light emitted from a light emitting element.
2. Description of the Related Art
Conventionally, adhesive (adhesion layer) of resin is used for mounting a light emitting element on a lead frame as mounting member. In some cases, transparent resin such as epoxy resin is used as adhesive since a component of light emitted from the light emitting element is discharged toward the lead frame and it is necessary to reflect that light component on the lead frame.
However, the adhesion layer composed of resin cannot sufficiently radiate heat generated from the light emitting element to the lead frame (i.e., it lacks in heat radiation property). Also, a light emitting device with adhesion layer of resin cannot be treated at high temperature in solder reflow oven (i.e., it lacks in heat resistance property).
Japanese patent application laid-open No. 07-086640 (paragraphs 16 and 21) discloses to use low melting point glass as adhesive (adhesion layer). Also, it discloses that an insulative filler with good thermal conductivity is blended into the adhesive to enhance its heat radiation property to radiate heat generated from light emitting element.
The other related art is disclosed in Japanese patent application laid-open No. 2001-214093.
The adhesion layer disclosed above is expected to have good heat resistance and heat radiation properties as compared to an adhesion layer only composed of resin. However, as the output of light emitting element increases rapidly, higher heat resistance and heat radiation properties are required to the adhesion layer.
On the other hand, a light emitting device is suggested that light emitted from a light emitting element is wavelength-converted by phosphor material to obtain a desired emission color.
Japanese patent application laid-open No. 2001-217466 discloses a light emitting device that its light emitting element is covered with sealing resin material (phosphor layer) with phosphor particles dispersed therein. However, since resin included in the phosphor layer has moisture-absorption characteristics, the phosphor is likely to degrade due to absorbed moisture. So, Japanese patent application laid-open No. 2001-217466 (paragraph 15 etc.) suggests to use an inorganic material instead of resin while dispersing phosphors therein. This can prevent the degradation of phosphor due to moisture.
The other related arts are disclosed in Japanese patent application laid-open Nos. 2001-214093, 2002-208733, 2002-203989 and 2002-134790, and Japanese patent Nos. 3307316, 3337000, 3230158 and 2924961.
However, the phosphor layer of conventional light emitting device has problems below.
When an inorganic material such as glass is used as the matrix of phosphor layer, it is very difficult to homogeneously disperse phosphor particles in the matrix since the fluidity of inorganic material is low even in softened state. Even if a low melting point glass is used as the inorganic material, the fluidity in softened state is still low. Therefore, the amount of phosphor particles to be blended into glass must be limited. Because of this, the phosphor layer needs to be thickened to secure the amount of phosphor particles required to obtain sufficient fluorescent light. In this case, there occurs a difference in an amount of received light per unit volume from the light emitting element between the lower layer (light emitting element side) and upper layer (light emission surface side) of phosphor layer. As a result, lights to be radiated from the upper layer and lower layer differ in wavelength spectrum distribution and unevenness in emission color may occur depending on observation direction.